In certain parts of the road network, and particularly in cities, a signaling system is necessary for regulating the traffic flow. For this purpose, for example, traffic lights can be controlled to give priority to traffic flow on one road section rather than on another, especially at intersections between minor roads and major highways or between roads having widely differing traffic patterns.
Normally, traffic-regulating systems have fixed repetition cycles. The resulting predetermined passage time for vehicles coming from a given road section can lead to congestion at the crossroads in question. Thus, traffic-flow conditions evolve differently over a 24-hour period on road sections leading to one and the same crossroads. It is therefore necessary to take account of these traffic variations on intersecting roads. Moreover, the increasing number of long and heavy vehicles of the bus and truck-and-trailer types make it necessary to modify the operating cycle of the traffic lights in such a way that when such a vehicle arrives at the intersection it can easily pass across it.
Devices known in the prior art make it possible to define certain characteristic parameters of the traffic flow on a given road section. In view of their measuring simplicity, the most frequently used means of this sort respond to the flow rate of vehicles per unit of time, the average speed of the vehicles passing a certain location at a given time, the concentration of the vehicles on a given road section and the extent of occupancy of vehicles traversing this section. The direct control of traffic lights, for example at a road intersection, as a function of the values obtained from measuring the aforementioned parameters leads to numerous disadvantages. Measurements have been taken in urban networks on road sections having widely differing traffic patterns. With comparisons based on measurements of speed, flow rate and concentration or occupancy level, for example, the mean value of the variations obtained for the traffic-signal control at the road sections involved was low. This shows that ambiguities exist, which can be of a highly prejudicial nature, if consideration is given to only one of the aforementioned parameters.